The technique of programmable frequency shifting of a Pierce crystal oscillator network is well understood in the art. This technique is widely used for clock generator chips (integrated circuits) and in applications where one system is tracked and another system is synchronized therewith. The Pierce oscillator functions as a resonant tank circuit, which comprises an active gain element, biasing elements, the resonating crystal and capacitive shunts for each crystal leg. For a crystal to resonate at the specified frequency of interest for a Pierce oscillator, certain loading criteria must be properly observed. Known as the Barkhausen effect, specified value ranges for gain and phase are applied to the system for oscillation to occur.
The oscillator frequency can be shifted or pulled through selection of loading capacitors. Different capacitance values results in slight phase changes due to changes in reactance. In order to maintain oscillation, subtle changes in phase require an equal offset in frequency to compensate for the difference.
Some conventional devices employing Pierce oscillators provide internal crystal capacitor selection. These capacitors are deployed within the device, e.g., an integrated circuit (IC) device, as an array of capacitive elements with a particular unit capacitance value. Each such element occupies, e.g., consumes, a certain “unit capacitance area.” Depending on the capacitive range of interest for the crystal, the silicon area required may add to a significant portion of the die area.
The internal capacitive weighting array typically comprises a selection of differently sized capacitors, e.g., with differing capacitance values. These capacitors can be added, or removed in parallel to appropriately load the crystal. Typically, such increments and decrements are partial capacitance units. For example, one such device provides a fractional capacitance range of 0.5 to 0.0625 pF. In order to maintain consistent manufacturing standards, the capacitors must be manufactured will little variance because as the capacitance value decreases, error margin increases.
Field effect transistor (FET) switching is typically used to enable a distinct capacitor array for each crystal leg. Typical architecture enables the capacitors in parallel mode. Several approaches are used in various applications for driving the FET switching, including analog-to-digital converter (ADC) technology such as for an analog control voltage controlled crystal oscillator (VCXO) type architecture. Another approach uses a variable capacitor, e.g., a varactor. Common convention is to introduce DC bias to the tank in order to modify the capacitance through DC voltage means.
An operative principle in these approaches is modification of the shunt reactance, which changes phasing relationships. Phasing changes lead to frequency shifting, which bring the system back into equilibrium. Other approaches include use of direct digital look up tables, binary weighting and other means appropriate for a particular application. The “state” of the capacitor selection, e.g., the capacitance value selected, remains static for a given frequency of interest. Where the varactor approach to driving FET switching is used, the “control” bias voltage typically remains stable for any given frequency of interest.
A conventional capacitive array 100 for shifting the oscillating frequency of crystal 110 of one typical approach is shown in FIG. 1. The crystal loading capacitors 111-119 and 131-139 (in practice, there can be any number of capacitors) are respectively switched by activating FETs 121-129 and 141-149. Other transistors or single pole single throw (SPST) switch can be used, represented herein with these FETs. The resolution of the target frequency depends on the accuracy and resolution of the available capacitor selection.
As seen in FIG. 1, typical conventional approaches use a number of capacitors on each crystal leg, each capacitor having a FET or other transistor, switch, etc. associated therewith. Silicon area is used for each capacitor configured within an IC. This can be a significant issue, because the silicon required may be significant, depending on the capacitive range of interest for a particular resonant crystal. Further, requiring a FET or other transistor, switch, etc. for switching each capacitor, as needed, can pose challenges related to control circuit and conductor routing complexity, reliability and cost.